you have to scrape them with SB. check there PR and then look if they are active or not.
takes a while and its very hard nowerdays to find better than pr2 because it is done by a lot of people in the meanwhile

I did this a lot in past, but after some time tumblr found out and deleted all my accounts so really do not know if it is worth the effort. It takes long time and you need good proxies and them after few weeks your blogs are gone. Better look for some more long term solution. All you need is scrapebox and some proxies, then you will use -site:"tumblr.com" inurl:tumblr.com and just few hundreds names list, after that you have big list of blogs you will just run alive check, then pr check and you are done.

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